The invention relates to subsea oil production systems and in particular to a connector for tying back a riser from a wellhead to a surface platform.
Offshore oil wells may be drilled from a floating platform and thereafter produced to a later constructed fixed or tethered platform. Such a procedure requires the running of tiebacks or risers from the platform deck to the wellhead in order to tieback the wellhead to the platform. Tubing is thereafter run, surface production trees installed, and the wells produced in a conventional manner.
The outermost conductor or riser must be connected and sealed in some manner to the wellhead. Particularly with the tensioned leg platform where the upper end of the riser is permitted to move horizontally, a bending moment is produced at the wellhead. This may occur even with the fixed platform where there is significant current force acting on the riser. The connection to the wellhead must also be capable of carrying substantial vertical force either in compression where insufficient load is carried by the platform or in tension where excessive load is carried by the platform.
Thermal expansion of various components of this structure also occurs depending on whether or not the well is producing at a particular time and the temperature of the fluid being produced. Furthermore as contrasted to the relatively short time period of drilling a well, the riser and its connection must endure these stresses through many cycles over many years.
One approach to making this tieback connection is illustrated in U.S. Pat. No. 4,343,495 wherein the riser funnel is locked to the wellhead housing and seals at the upper end of the housing. This has a single seal location and while the structure can be made very rigid, there can still be slight movements at the seal location which over a period of years would lead to seal failure. Furthermore, the casing hanger packoff is exposed to the fluid and pressure within the inner downhole casing string thereby resulting in possible deterioration of the casing hanger packing.
In another approach to a tieback connection, illustrated in co-pending application Ser. No. 241,187, now U.S. Pat. No. 4,408,784 issused Oct. 11, 1983, the downwardly-extending funnel surrounds the wellhead housing for purposes of limiting deflection while a floating bushing ties the stab pin to the casing hanger. This again has a single seal location which must accept the repeated bending strain up to the limit provided by the housing-funnel interaction. This connection with the bushing and seal ring is also subject to the entire range of tensile and compressive loads placed on the connector. Any vertical upward loading passing through the riser and bushing to the casing hanger is additive with any expansion forces tending to push the hanger up thereby increasing the difficulty of retaining the hanger at its packed off location with the wellhead housing.
In each of the prior art schemes, the seal location is immediately adjacent the direct load path for both bending and tension; and any attempt to provide a secondary seal is plagued with problems because of fabrication tolerance and movement of the various members.